Glycolysis

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Glycolysis
Glycolysis is the sequence of reactions that
metabolizes one molecule of glucose to two
molecules of pyruvate with the concomitant net
production of two molecules of ATP - energy The
process is anaerobic, O2 is not required - an
ancient pathway. Pyruvate is further
processed Anarobically through
fermentation, Aerobically by complete oxidation
to CO2, generating more ATP
Chapter 16 Outline
16.1 Glycolysis is an energy-conversion pathway
in many organisms 16.2 The glycolytic
pathway is tightly controlled
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Understanding glycolysis - rich in history
1897 Hans Eduard Buchner, attempting to
manufacture cell- free extracts of yeast for
possible therapeutic use, added sucrose as a
preservative, and discovered that the sucrose was
fermented into alcohol by the yeast juice!!! The
significance of this finding was immense. The
Buchners demonstrated for the 1st time that
fermentation could take place outside living
cells. It refuted the 1860 assertion of Louis
Pasteur, that fermentation is inextricably tied
to living cells. The vitalistic dogma was gone,
and metabolism became chemistry
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History continued
Studies of muscle extracts then showed that many
of the reactions of lactic acid fermentation were
the same as alcohol fermentation. This discovery
revealed an underlying unity of
biochemistry. 1940 the complete glycolytic
pathway was elucidated. Glycolysis is also known
as the Embden-Meyerhof pathway, from the
pioneering work of Gustav Embden Otto Meyerhof
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Some fates of glucose
Glucose is an important fuel for most organisms
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Fermentations
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Stages of Glycolysis
Three stages
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Glycolysis 1
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Glycolysis 2
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Glycolysis 3
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Glycolysis stage 1
The three steps of stage 1 begin with the
phosphorylation of glucose by hexokinase
Energy used, none extracted
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Glucose phosphorylation step 1
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Induced fit in hexokinase
Conformation changes on binding glucose, the two
lobes of the enzyme come together and
surround the substrate
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Formation of fructose-6-phosphate step 2
by phosphoglucose isomerase
Conversion of an aldose to a ketose
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Formation of fructose 1,6-bisphosphate step 3
by phosphofructokinase (PFK) an allosteric
enzyme that regulates the pace of glycolysis.
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Glycolysis stage 2
Two 3-carbon fragments are produced from
one 6-carbon sugar
No energy used or extracted
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Cleavage of six-carbon sugar step 4
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Salvage of three-carbon fragment step 5
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Triosephosphate isomerase
Central core of 8 parallel ? strands, surrounded
by 8 ? helices. This structural motif, called an
?? barrel,is also found in three
other glycolytic enzymes.
The loop closes off the active site on
substrate binding
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Glycolysis stage 3
The oxidation of three-carbon fragments yields ATP
Energy extracted, 2x2 ATP
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Formation of 1,3-Bisphosphoglycerate step 6
Done in two steps
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Two-process reaction
Aldehyde
Acid
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Glyceraldehyde 3-phosphate dehydrogenase
Active site configuration
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Formation of ATP from 1,3-Bisphosphoglycerate
step 7
High phosphoryl- transfer potential
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Rearrangement step 8
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An enol phosphate is formed step 9
Dehydration elevates the transfer potential of
the phosphoryl group, which traps the molecule in
an unstable enol form
Enol molecule with hydroxyl group next to double
bond
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Formation of Pyruvate ATP step 10
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Maintaining Redox Balance
NAD must be regenerated for glycolysis to proceed
Glycolysis is similar in all cells, the fate of
pyruvate is variable
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Diverse fates of pyruvate
To citric acid cycle
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Lactobacillus intestine, urogenital tract, food
industry
Ferment glucose to lactic acid
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Glycolytic pathway tightly controlled

• Pathway is regulated to meet two major needs
• Production of ATP
• Provision of building blocks for biosynthesis

• Hexokinase, phosphofructokinase, pyruvate
  kinase serve as
• control sites (their reactions are virtually
  irreversible)
• Their activities are regulated by reversible
  binding of allosteric
• effectors (milliseconds), or by covalent
  modification (seconds)
• The amounts of the enzymes are varied by the
  regulation of
• transcription (hours)

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Phosphofructokinase
Key enzyme in the control of glycolysis
Allosteric regulation
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Allosteric regulation of phosphofructokinase
(step 3)
Decreases affinity for F 6-phosphate
AMP diminishes, citrate enhances the inhibitory
effect of ATP
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Phosphofructokinase regulation of activity (step
3)
activation by fructose 2,6-bisphosphate
hyperbolic
ATP inhibition reversed
sigmoidal
Allosteric activator, shifts conformational equili
brium from the T to the R state
Also regulated by pH to prevent acidosis